1. Field of the Invention
The present invention relates to effective technique for an object detector using a phased array antenna or the like.
2. Description of the Prior Art
Technique for detecting an object by irradiating an electric wave to predetermined regions and analyzing an echo reflected by an object existing in the detection region is well known. Such remote sensing technique using the electric wave is applied to an object detector such as a detector of an intruder for security or a car detector for a vehicle, for example.
FIG. 10 shows an example of an object detector 100 using a phased array antenna. The phased array antenna comprises an array antenna having a plurality of arranged antenna elements, and a plurality of phase shifters each controlling a feeding phase to each antenna element. The phased array antenna can obtain a desired beam configuration by using a directional synthesis of each element, and can switch a beam direction electronically by differentiating a feeding phase every element.
According to the illustrated example, an antenna beam width (main lobe width) of about 10 degrees is implemented using eight antenna elements and beam scanning is performed in five directions every 10 degrees with 0 degree set at the front as a center thereof. In FIG. 10, regions designated by A to E are detection regions in the respective directional characteristics (beam directions).
When an object is detected, a feeding phase is set such that the beam direction is oriented to the detection region A first, and the electric wave is irradiated to the detection region A and its echo is observed. When the object exists in the detection region A, a power intensity of the echo (reception power intensity) becomes strong. Therefore, the existence of the object in the detection region A can be detected by determining whether the reception power intensity exceeds a predetermined threshold value or not. Similarly, the detection regions B to E are sequentially scanned, so that object detection can be performed within a range of 50 degrees.
According to this kind of object detector, since it is necessary to differentiate the detection object from another object and to specify an accurate bearing of the object, excellent directional characteristics is demanded as antenna characteristics. In addition, it is desirable to be able to finely vary the beam direction in order to improve bearing resolution.
Although the scanning is performed in five directions every 10 degrees with the beam width of 10 degrees in the above example, when the higher bearing resolution is required, the beam width is narrowed by increasing the number of antenna elements and scanning is performed in wide range of angles by setting the beam angle more finely.
When the number of antenna elements is increased in order to implement sharp directional characteristics, it becomes necessary to add a phase shifter or a circuit in accordance with that, which increases its cost. In addition, since an antenna area is increased, the detector itself is enlarged. According to the object detector for security or vehicle, since a compact detector is needed as much as possible in view of restriction of setting, the above problems are serious especially.
When the method of the JP Publication No. 08-105955 is employed, although the antenna area is not enlarged, two types of phase shifter circuits for transmission and reception are required and also phase shift value setting with high precision is required, causing the phase shifter circuit to be complicated. Therefore, practical application is difficult at a consumer level.
Furthermore, when the beam direction is controlled finely over wide angles, very high-performance phase shifter is required, and designing and manufacturing of the antenna or a line length become severe. Consequently, there are provided problems that the constitution becomes complicated, its yield is lowered, its cost is increased and the like. In addition, when the continuous scanning is implemented as in the JP Publication No. 07-106839 it is necessary to provide a compensation circuit or the like in order to prevent a characteristic change due to temperature variations of the phase shifter and the amplifier, and also the control with high precision is required, causing the constitution to be complicated and the cost to be increased.